Extra-framework Aluminum Research Articles

Nature and Catalytic Role of Extraframework Aluminum in Faujasite Zeolite: A Theoretical Perspective

A comprehensive periodic DFT study complemented by ab initio thermodynamic analysis was carried out to determine the speciation of extraframework aluminum (EFAl) in faujasite zeolite. The structure and stability of a wide range of mono- bi-, tri-, and tetranuclear EFAl complexes stabilized at different locations in faujasite were investigated. The thermodynamic cycles connecting these complexes were constructed involving such elementary steps as hydration/dehydration, proton transfer, and condensation reactions. Using ab initio thermodynamics analysis it was predicted that, during high-temperature zeolite activation, the EFAl species self-organize into cationic clusters with more than one Al center. The resulting tri- and tetranuclear clusters are preferentially stabilized inside the small sodalite cages of faujasite that provide a favorable coordination and charge-compensation environment for the large multiply charged cationic clusters. The presence of such cationic EFAl clusters inside the inaccessible...

Promotion effect of mesopore on the conversion of carbohydrates to methyl levulinate over H-USY zeolite

H-USY was treated with nitric acid solution to remove Al species and thus adjust the mesoporosity and acidity. Effects of mesopore and acidity of H-USY zeolite were studied on the conversion of carbohydrate to methyl levulinate (MLE). Low concentration of nitric acid mainly removed the extraframework aluminum species with the increase of mesoporosity and resulted in a slight decrease of acidity. Both extraframework and framework Al species were removed to some extent under high concentration of nitric acid, associating with the obvious decrease of acidity. H-USY treated with low concentration of nitric acid showed higher yield of MLE than H-USY-parent. Generated mesopores improved the diffusion limitations, facilitated the accessibility of reactant to the acid sites, and thus promoted the formation of MLE.

Organic-Free Synthesis of CHA-Type Zeolite Catalysts for the Methanol-to-Olefins Reaction

Chabazite (CHA)-type zeolites are prepared from the hydrothermal conversion of faujasite (FAU)-type zeolites, dealuminated by high-temperature steam treatments (500–700 °C), and evaluated as catalysts for the methanol-to-olefins (MTO) reaction. The effects of temperature and partial pressure of water vapor during steaming are investigated. Powder X-ray diffraction (XRD) and Ar physisorption data show that the steam treatments cause partial structural collapse of the zeolite with the extent of degradation increasing with steaming temperature. 27Al MAS NMR spectra of the steamed materials reveal the presence of tetrahedral, pentacoordinate, and octahedral aluminum. NH3 and i-propylamine temperature-programmed desorption (TPD) demonstrate that steaming removes Bronsted acid sites, while simultaneously introducing larger pores into the CHA materials that make the remaining acid sites more accessible. Acid washing the steamed CHA-type zeolites removes a significant portion of the extra-framework aluminum, prod...

Brønsted sites and structural stabilization effect of acidic low-silica zeolite A prepared by partial ammonium exchange

It is demonstrated for low-silica zeolite 25NH4,Na-A (nSi/nAl = 1.0) with an ammonium exchange degree of 25% that the formation of acidic bridging OH groups (Si(OH)Al) after thermal deammoniation is accompanied by a weak dealumination of the framework. As the extra-framework aluminum species mainly exist predominantly in the form of Al3+ cations, they partially neutralize negative framework charges. The remaining framework charges are compensated by Na+ cations and hydroxyl protons of Si(OH)Al groups. Therefore, a lower number of Si(OH)Al groups is formed than expected by the stoichiometry of the ammonium exchange. For zeolite 25NH4,Na-A, only 17% of the introduced ammonium ions form Si(OH)Al groups after thermal deammoniation. A benefit of the cationic extra-framework aluminum species (Al3+) is their stabilizing effect exerted on the remaining framework aluminum atoms and the whole structure of the obtained zeolite 25H,Na-A. 1H MAS NMR signals of acidic OH groups were observed at 3.6 and 4.8 ppm and assigned to Si(OH)Al groups in the α- and β-cages, respectively. A low-field signal at 13.3 ppm is probably due to Si(OH)Al groups, however, involved in H-bondings (Si(OH⋯O)Al) with hydroxyl protons directed to very near oxygen atoms, such as expected in double 4-rings of the LTA structure.

Comparison of the defluoridation capacity of zeolites from Ethiopia and Mexico

Fluorosis, either dental or skeleton, is often due to the high fluoride content of well waters. In this work, using solutions which contain different amounts of fluoride, natural zeolites from Ethiopia and Mexico were tested. It is shown that, although zeolites are known to be cationic exchangers, their extra-framework aluminum and their high calcium contents determine their performance. A mechanism involving adsorption and ion exchange is proposed. KEY WORDS: Ion exchange, Purification, Fluorosis, Clinoptilolite, Stilbite, Heulandite, Analcime Bull. Chem. Soc. Ethiop. 2015, 29(1), 53-62DOI: http://dx.doi.org/10.4314/bcse.v29i1.5

Hydrophilicity of Mordenites with Different SiO2/Al2O3 Molar Ratio

Hydrophicility of H-mordenites with SiO2/Al2O3 molar ratio (MR) varying in the range from 10 up to 206 was investigated by IR spectroscopy, TPD of ammonia and thermogravimetry methods. It was found that dealuminated mordenite retains affinity for water in the entire MR range, in contrast to published results for synthetic pentasil zeolites. In H-mordenites extra-framework aluminum exists in the form of dimers of aluminum hydroxide, those presumably are stabilized in the channels of mordenite. An inversely proportional correlation between the water content in pores and the concentration of channel Lewis sites was observed.

Conceptual Frame Rationalizing the Self-Stabilization of H-USY Zeolites in Hot Liquid Water

The wide range of liquid-phase reactions required for the catalytic conversion of biomass compounds into new bioplatform molecules defines a new set of challenges for the development of active, selective, and stable catalysts. The potential of bifunctional Ru/H-USY catalysts for conversions in hot liquid water (HLW) is assessed in terms of physicochemical stability and long-term catalytic performance of acid sites and noble metal functionality, as probed by hydrolytic hydrogenation of cellulose. It is shown that zeolite desilication is the main zeolite degradation mechanism in HLW. USY zeolite stability depends on two main parameters, viz., framework and extra-framework aluminum content. The former protects the zeolite lattice by counteracting hydrolysis of framework bonds, and the latter, when located at the external crystal surface, prevents solubilization of the zeolite framework which is the result of its low water-solubility. Hence, the hot liquid water stability of commercial H-USY zeolites, in cont...

Aluminum-phosphate binder formation in zeolites as probed with X-ray absorption microscopy.

In this work, three industrially relevant zeolites with framework topologies of MOR, FAU and FER have been explored on their ability to form an AlPO4 phase by reaction of a phosphate precursor with expelled framework aluminum. A detailed study was performed on zeolite H-mordenite, using in situ STXM and soft X-ray absorption tomography, complemented with (27)Al and (31)P magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, XRD, FT-IR spectroscopy, and N2 physisorption. Extraframework aluminum was extracted from steam-dealuminated H-mordenite and shown to dominantly consist of amorphous AlO(OH). It was found that phosphoric acid readily reacts with the AlO(OH) phase in dealuminated H-mordenite and forms an extraframework amorphous AlPO4 phase. It was found that while AlPO4 crystallizes outside of the zeolitic channel system forming AlPO4 islands, AlPO4 that remains inside tends to stay more amorphous. In the case of ultrastable zeolite Y the FAU framework collapsed during phosphatation, due to extraction of framework aluminum from the lattice. However, using milder phosphatation conditions an extraframework AlPO4 α-cristobalite/tridymite phase could also be produced within the FAU framework. Finally, in steamed zeolite ferrierite with FER topology the extraframework aluminum species were trapped and therefore not accessible for phosphoric acid; hence, no AlPO4 phase could be formed within the structure. Therefore, the parameters to be taken into account in AlPO4 synthesis are the framework Si/Al ratio, stability of framework aluminum, pore dimensionality and accessibility of extraframework aluminum species.

Investigation of Aluminum Site Changes of Dehydrated Zeolite H-Beta during a Rehydration Process by High-Field Solid-State NMR

Aluminum site changes of dehydrated H-Beta zeolite during a rehydration process are systematically investigated by 27Al MAS and MQ MAS NMR at a high magnetic field up to 19.9 T. Benefiting from the high magnetic field, more detailed information is obtained from the considerably broadened and overlapped spectra of dehydrated H-Beta zeolite. Dynamic changes of aluminum sites are demonstrated during the rehydration process. In completely dehydrated H-Beta, invisible aluminums can reach 29%. The strength of quadrupole interactions for framework aluminum sites decreases gradually during the progressive water adsorption process. Extra-framework aluminum (EFAL) species, that is, penta- (34 ppm) and octahedral- (4 ppm) coordinated aluminum atoms, rise initially with increasing amount of water adsorption, and finally change into either tetra-coordinated framework or extra-framework aluminum in water saturated samples, with the remaining octahedrally coordinated aluminum lying at 0 and −4 ppm, respectively. Quantit...

Optimization of catalytic hydrodeoxygenation of oleic acid into biofuel using fluoroplatinum oxalate zeolite supported catalyst

Optimization of hydrodeoxygenation (HDO) of oleic acid (OA) into paraffinic biofuel using zeolite supported fluoroplatinum oxalate (FPtOx/Zeol) catalyst is reported in this study. FPtOx/Zeol was synthesized via incorporation of HF modified oxalic acid (OxA) functionalized K2PtCl4 into zeolite A support. FPtOx/Zeol was characterized and the results showed highly dispersed tetrahedral and octahedral Pt species which are effective for sequential hydrogenation of the unsaturated OA bond and the HDO of OA, respectively. Furthermore, there was increase in Si/Al ratio of FPtOx/Zeol leading to loss of crystallinity due to dealumination of the extra framework and framework alumina as a result of acid attacks and calcination, respectively. FPtOx/Zeol activity was tested on the HDO of OA in a semi-batch reactor and the process parameters were optimized using validated quadratic models. The optimum conditions were 364°C, 18bar and 27.8mg FPtOx/Zeol loading under reaction time of 58min to produce 28.39% iso-C18H38 and 68.93% n-C18H38. The presence of the isomerized product was due to the combine effect of fluoride ion and oxalic acid functionalization on the catalyst. FPtOx/Zeol reusability studies showed consistency after three consecutive runs. These results are promising for further research toward commercialization of biofuel production.

On the effect of EDTA treatment on the acidic properties of USY zeolite and its performance in vacuum gas oil hydrocracking

USY zeolite catalysts were chemically modified by treatment with aqueous solutions of sodium ethylenediaminetetraacetic acid (EDTA) to study the effect of removal of extraframework aluminum species (EFAl) on hydrocracking performance. The textural, structural and acidic properties were characterized by XRD, elemental analysis, XPS, N2 physisorption, HRTEM, 29Si and 27Al solid-state NMR, ammonia TPD and IR spectroscopy of adsorbed pyridine. The acid catalytic activity of the zeolites was measured by the conversion of propane and 1,3,5-tri-isopropylbenzene. EDTA is a selective agent for the extraction of EFAl from USY preserving the framework Al content. Concomitant with EFAl removal, EDTA enhances the textural properties and changes bulk and surface properties, in particular the Al speciation. Hydrocracking catalysts were prepared from the modified USY zeolites and alumina loaded with a P-promoted NiMo-sulfide phase. Their performance was evaluated in the hydrocracking of a heavy vacuum gas oil under conditions close to the industrial practice. The most active hydrocracking catalyst is obtained by removal of a relatively small amount of EFAl from the USY zeolite. The mild EDTA treatment to achieve this selectively extracts amorphous EFAl species from the mesopore system, resulting in better access to acid sites and an increase of the concentration of strong Brønsted acid sites. Severe EDTA treatment resulted in lower hydrocracking activity which was ascribed to progressive leaching of EFAl synergistically involved in the cracking catalysis. The present study therefore points out the importance of the presence of some EFAl species for the hydrocracking of real feedstocks. This work also shows that even with supports with increased mesoporosity and stronger acid character, higher hydrocracking activity of a real feedstock is not guaranteed.

Acid properties and catalysis of USY zeolite with different extra-framework aluminum concentration

USY zeolite was treated by the citric acid. The properties and catalysis of the USY zeolite were studied. The X-ray diffraction has been used to identify and quantify extra framework aluminum (EFAL) in USY zeolite by EFAL extraction using citric acid. The acid character changed depending on the EFAL concentration. The Lewis acid sites can be obtained from the Si/Al ratio. The removing of trace olefins was carried out over kinds USY zeolites with different amount of EFAL. The catalytic performance was correlated with the Lewis acid sites of USY.

Modification of acid sites in ZSM-5 by ion-exchange: An in-situ FTIR study

A study of the acid sites in Mn+-ZSM-5 zeolites (Mn+=H+, Al3+, Ca2+, and Ba2+), synthesized by ion-exchange from the NH4+ form, has been carried out using X-ray diffraction, 27Al MAS NMR, and in-situ FTIR spectroscopy. X-ray diffraction data indicate that the crystalline structure of ZSM-5 is maintained on ion-exchange. 27Al MAS NMR spectroscopy confirms that Lewis acid sites in all of the Mn+-ZSM-5 zeolites are mainly located on the tetrahedral aluminum atoms in the zeolite framework. However, octahedral extra-framework aluminum is another source of Lewis acid sites in H+-ZSM-5 and Al3+-ZSM-5. Brønsted acid sites are identified as originating from the hydroxyl groups that bridge Al and Si atoms. The acid sites in Mn+-ZSM-5 have been characterized by in-situ FTIR spectroscopy with pyridine as the probe molecule. FTIR spectroscopy demonstrated that the number of accessible acid sites and ratio of Lewis to Brønsted acid sites in Mn+-ZSM-5 can be effectively modified by ion-exchange. The number of acid sites in Mn+-ZSM-5 increases in the same order as the acidity of cations with Ba2+<Ca2+<NH4+<Al3+<H+. Though the strength of both Lewis and Brønsted acid sites is virtually identical for all Mn+-ZSM-5 zeolites, the ratio of Brønsted to Lewis acid sites varies as a result of the loaded cation. In addition, modification of acid sites in Mn+-ZMS-5 by loading different cations is discussed in terms of the accessibility of the surface of the zeolite channels and the reactivity of the cations with Brønsted acid hydroxyls. We conclude that larger cations can effectively constrict the zeolite channels and impede the ability of pyridine to access acid sites on the surface of the channels.

Generation of synergistic sites by thermal treatment of HY zeolite. Evidence from the reaction of hexane isomers

We have studied the conversion of hexane isomers on HY zeolites (Si/Al=40 and 2.6) after different thermal pretreatments (400–600°C) on a micropulse reactor, which is ideal for evaluating catalyst activity and evolution of product distribution over fresh zeolites at very short times on stream, when coke formation is negligible. We have observed that the catalytic activity of the zeolites is enhanced upon high-temperature pretreatment. This activity enhancement can be ascribed to dehydroxylation, as observed by thermogravimetric analysis upon heating the zeolite above 500°C. However, this dehydroxylation does not result in a decrease in the density of the original Brønsted acid sites (BAS). Therefore, the newly created Lewis acid sites (LAS) must be associated with either extra-framework alumina (EFAl) or silanol species. We propose that the activity enhancement is associated with the generation of synergistic sites resulting from the interaction between LAS created by the dehydroxylation and the original BAS. Both, protolytic cracking and dehydrogenation reactions are enhanced with paraffin feeds, but such enhancement is not apparent for olefin feeds. The synergistic sites were also deactivated very quickly by strong adsorption of hydrocarbon species, but could be regenerated by a subsequent high-temperature treatment.

The variety of Brønsted acid sites in amorphous aluminosilicates and zeolites

Brønsted acid sites of similar strength to those in zeolites are found in ASAs prepared by cogelation, as shown by thermogravimetric study of the dehydration of ethanol to ethylene. ASAs prepared by grafting and a commercial ASA (Siralox 30) show Brønsted acid sites that differ from those of zeolites. CO respectively ethanol adsorption followed by infrared spectroscopy identified the OH stretching frequency of the Brønsted acid sites specific to Si/Al2O3 and Al/SiO2 materials obtained by grafting to be ca. 3600cm−1 and 3640cm−1, respectively, although the intensity of the corresponding band is weak. These sites, contrary to those of a H-USY zeolite with very few extra-framework aluminum, are highly sensitive to the surface hydration state: Their number decreases with increasing temperature. The surface structure of Siralox 30 resembles that of Si/Al2O3 prepared in anhydrous conditions, showing similar isosurface acidic properties and distribution of silicon species.

ESR and DFT study of the paramagnetic carbon centers stabilized in γ-irradiated zeolites exposed to carbon monoxide

Free radicals are important transient species in many chemical reactions, especially in catalysis, photochemistry and radiation chemistry. However, the identification of free radicals is difficult because of their high reactivity. For catalytic systems the EPR study of radiation-induced free radicals at low temperature brings new insights to better understand many real catalytic processes.In this article we present an EPR study on carbon-centered radicals formed in γ-irradiated ZSM-5, MOR, FAU and LTA zeolites exposed to 13CO (S=1/2). Four different types of carbon-centered radicals were detected. In zeolites with high Si content such as ZSM-5, FAU Y and MOR, three types of radical centers are stabilized: center A [SiO−Al]+CO, center A′ [SiOSi]+CO and center B [SiO]CO. In low-siliceous zeolites only center A is recorded. The hyperfine splitting Aav[13C] values for A and A′ centers are changing in the range 24.7–29.3mT depending on Si/Al ratio and zeolite structure. These differences are caused by subtle changes in geometry of carbon centers occurring even in zeolite frameworks belonging to the same topological group. On the other hand, the hyperfine splitting of center B is changing only in narrow range 21.0–21.4mT, indicating that magnetic properties of this center weakly depend on geometry. Additionally, in ultra stable Y zeolite (USY) a carbon radical center with exceptionally high Aav[13C] value (Ax=35.9mT, Ay=32.3mT, Az=32.3mT) has been observed. We assigned it to the +CO radical cation which is located in the immediate vicinity of EFAL (extra framework aluminum) species. This carbon radical is very unstable and disappears completely above 120K.

Cooperativity of Brønsted and Lewis Acid Sites on Zeolite for Glycerol Dehydration

Selective dehydration of glycerol, a byproduct from the biodiesel industry, on solid acids is an important reaction in the production of the value-added chemical acrolein for economic-sustainable biorefinery. Most efforts have been made on the development of strong Brønsted acid sites (BAS) to improve the production of acrolein, because the Lewis acid sites (LAS) generally promote the generation of the byproduct acetol. However, exclusively tuning the properties of BAS or LAS did not well-promote the acrolein production from glycerol as indicated in this work. We provide a new route for efficient and selective glycerol transformation to acrolein via the cooperative dehydration between the BAS and LAS. The role of LAS (extra-framework aluminum species on zeolites) was altered from competition with BAS to generate the byproduct acetol to cooperation with the neighboring BAS. It is very beneficial for the sequential two-step dehydration of the internal and terminal hydroxyl groups of glycerol to value-added acrolein. This cooperativity of BAS and LAS significantly improved the yield of acrolein from the selective glycerol dehydration.

Localization of the copper-containing component in the pore volume of zeolite ZSM-5

The distribution of the copper-containing component in the pore volume of zeolite ZSM-5 has been investigated by H2 and N2 adsorption at 77 K and IR spectroscopy. Samples were synthesized by ion exchange and incipient wetness impregnation. Copper-containing clusters are mostly located on the surface of the mesopores formed by packed zeolite nanocrystallites. This causes partial blocking of the volume of microporous channels for N2 molecules, but these channels remain accessible for H2 molecules. It has been deduced that no considerable amount of copper located in the structural channels of the zeolite. According to IR spectroscopic data, the sorption of copper ions in the Cu/ZSM-5 catalysts takes place on extraframe-work aluminum, which forms Al-OH-Al bridges and terminal Al-OH groups, and on terminal Si-OH groups located on the zeolite crystal surface.

A Single-Event MicroKinetic assessment of n-alkane hydroconversion on ultrastable Y zeolites after Atomic Layer Deposition of alumina

The acid strength of a commercial Y zeolite subjected to Atomic Layer Deposition (ALD) of alumina using alternating pulses of trimethylaluminium (TMA) and water was assessed by means of Single-Event MicroKinetic modelling of n-decane hydroconversion. Catalytic activity changes were rationalized in terms of changes in three catalyst descriptors: the physisorption saturation concentration, the total Brønsted acid site concentration, and the average acid strength of these sites. The former two descriptors were measured independently from N2 adsorption, and pyridine TPD and FT-IR, while the latter was determined as the reference alkene standard protonation enthalpy by model regression. Changes in aluminium content were characterized by means of 27Al MAS NMR. Physico-chemically reasonable differences in sorbate stabilization by Van der Waals interactions with the zeolite framework could not explain the observed differences in n-decane hydroconversion performance between the ALD-treated USY samples. Instead, an increase in catalytic activity by the ALD treatment could be assigned to the creation of new and stronger Brønsted acid sites in the zeolite micropores exhibiting alkene standard protonation enthalpies that were up to 7kJmol−1 more negative on the ALD-treated samples compared to the parent zeolite. The newly formed acid sites were associated with extraframework aluminium species in a distorted tetrahedral configuration and were predominantly obtained using long TMA pulse times and a large number of ALD cycles provided that water was effectively removed from the ALD reaction chamber prior to ALD.

Hierarchical porous zeolite ZSM-58 derived by desilication and desilication re-assembly

Hierarchical porosity in zeolite ZSM-58 was achieved by desilication with pure solutions of NaOH and mixtures of NaOH and cetyltrimethylammonium bromide (CTAB) in a desilication and desilication re-assembly process. The resulting ZSM-58 samples were investigated concerning their chemical and textural structure by means of transmission and scanning electron microscopy, X-ray diffraction, nitrogen and argon physisorption, elemental analyses, 27Al and 29Si solid-state magic angle spinning nuclear magnetic resonance spectroscopy, and temperature programmed desorption of ammonia. In the case of applying pure NaOH solutions on Si/Al ratios of Si/Al=50 rather macroporous zeolite ZSM-58 with broad pore size distribution was obtained and pore volume depended on the sodium hydroxide concentration. Higher Si/Al ratios did not result in any additional porosity. Adding CTAB to the desilication solution lead to hierarchical ZSM-58 materials with narrow additional pores in the range of 3–4nm resulting from an amorphous layer on the surface of the ZSM-58 crystals that was long range ordered for intermediate NaOH concentrations independent from the initially deployed Si/Al ratio. The total specific surface area of such materials was as high as 868m2g−1 with high values of external specific surface area and simultaneously the desilication yield could be increased due to the re-assembly of dissolved silica species. Therefore, high values for the hierarchy factor and desilication efficiency could be achieved. The amount of strong acid sites of the desilicated samples decreased with increasing NaOH concentration because of the generation of silanol groups, pentahedral, and octahedral coordinated extra framework aluminum.